Fix PG intersects expression to avoid NPE

[rpanchyk]

This PR fixes NullPointerException:

ERROR [ntloop-thread-4] o.p.s.handler.openrtb2.AuctionHandler : Critical error while running the auction
java.lang.NullPointerException: null
at org.prebid.server.deals.targeting.interpret.IntersectsStrings.toLowerCase(IntersectsStrings.java:23)
at org.prebid.server.deals.targeting.interpret.IntersectsStrings.lookupActualValues(IntersectsStrings.java:19)
at org.prebid.server.deals.targeting.interpret.Intersects.matches(Intersects.java:25)
at org.prebid.server.deals.targeting.interpret.And.matches(And.java:21)
at org.prebid.server.deals.TargetingService.matchesTargeting(TargetingService.java:70)

It was occurred when FPD is not defined in request:

"site": {
  "publisher": {
    "id": "ACCOUNT"
  }

Until adding site FPD:

"site": {
  "publisher": {
    "id": "ACCOUNT"
  },
  "ext": {
    "data": {
      "key1": [ "value" ]
    }
  }
}

For PG targeting like:

"sfpd.key1": {
    "$intersects": [
        "value",
        "value2"
    ]
}

[And1sS]

Why not return Collections.emptyList() here and leave previous implementation of matches?

[rpanchyk]

NULL is more optimized. If look forward in org.prebid.server.deals.targeting.interpret.Intersects#matches method - the call of Collections.disjoint(..) is not required. Another though - lookupActualValues(..) method returns values in request, so empty list can real value.